Described below is a method for producing microalloyed tubular steel in a combined casting-rolling installation, and to the microalloyed tubular steel which can be produced by applying the method.
Microalloyed steels are those to which 0.01 to 0.1 percent by mass of niobium, vanadium or titanium (or even higher contents of the cited elements in combination) is alloyed in order to achieve high strength, e.g. by forming carbides and nitrides and grain refining. The alloying elements disperse partially during heating to deformation temperature. They are dispersed in the austenite and result in an increase in the recrystallization temperature. During the transformation of the shaped austenitic grain into ferrite and pearlite, bainite or even martensite, a refined transformation microstructure is formed according to the degree of deformation. In addition, carbides form in the case of carbon and nitrides form in the case of nitrogen during cooling. The grain refining achieved in this way increases the strength, essentially without reducing the toughness.
The yield point is a characteristic material value and denotes the maximal stress which a material will withstand before exhibiting any visible plastic deformation when subjected to a simple and zero-torque tensile load. If the yield point is exceeded, the material no longer returns to its original shape when the load is reduced, and an extension of the specimen remains. The yield point is generally determined during the tensile test and specified in the unit N/mm2=MPa.
Steel grades having a minimal yield point of at least 485 N/mm2 are required for the API 5L-X70 and API 5L-X80 standards (or higher) of the American Petroleum Institute (API). For example, steel according to the standard API 5L-X70 has a yield point of at least 485 N/mm2 and a tensile strength of at least 570 N/mm2.
This standard is used to designate steel products which can be used in the production of tubes by welding for the purpose of pipeline construction, and which must therefore have a specific strength but must also be highly ductile at low temperatures. The production of such steel products requires specific types of methods.
Existing manufacturing methods for such steels usually require the following conditions to be satisfied:
Only pure iron with low sulfur and phosphorous content is suitable for the steel composition, and the addition of microalloying elements such as niobium Nb, titanium Ti or vanadium V is required in order to achieve a fine grain.
Suitable temperature management is required in order to prevent segregation (dissociation) of the melt during the production of the slab. This occurs at the transition point of the melt into the solid state and results in different material properties within the slab. Suitable temperature management can also be used to prevent cracking in the second ductility minimum.
The rolling of the slab is effected by so-called thermomechanical rolling:
During roughing, a uniformly heated slab having a relatively coarse-grained microstructure arrives from a heating furnace at the roughing train (roughing mill). In order to achieve recrystallization, during which the relatively coarse-grained microstructure of the slab becomes increasingly fine-grained, the thickness of the steel must be reduced by at least 40%, normally in reversing operation. A fine austenitic grain is required before the rolled stock is shaped further in the finishing train.
During finish-rolling in the finishing train, final shaping takes place in a temperature range at which the material no longer recrystallizes: typically in the temperature range of 800-900° C. In this case, the steel band is stretched by a factor of at least 2.5 and usually more than 3 in order to achieve the desired material properties. Typical intake temperatures of the steel band into the finishing train are between 800° C. and 900° C. The exit temperature of the steel band emerging from the finishing train is typically in the region of 830° C.
Faster cooling of the steel band after it emerges from the finishing train results in the formation of particularly fine ferrite grains, even the development of acicular (needle-shaped) ferrite in some circumstances. This produces an extremely fine-grained transformation microstructure having high strength and very good toughness.
EP 1 978 121 A1 discloses a corresponding method for the production of highly weldable steel sheets having a yield point (yield stress) of at least 350 MPa and a tensile strength of at least 570 MPa, wherein the steel is thermomechanically rolled in the finishing train.
The production of API X70 tubular steel in a combined casting-rolling installation known as “CSP flex” is disclosed in the publication by C. Klein et al.: “Von CSP zu CSP flex—das neue Konzept für die Dünnbrammentechnologie, stahl and eisen 131(2011), No. 11”. The proposed production method has the disadvantage that the steel melt must have a relatively high proportion of alloy, in particular niobium, in order to limit the grain growth of the cut thin slab strand in the heating furnace (usually a tunnel furnace). The production costs per ton of finished strip increase due to the high proportion of alloy.
The existing methods have the disadvantage that undesired grain growth can occur due to the long time the steel spends in the heating furnace ahead of the roughing train and/or the finishing train.